1. Field of the Invention
The present invention is directed toward measuring thin films and more particularly toward measuring lubricant thickness, lubricant degradation, thin film thickness and wear, and surface roughness using a laser directed toward a thin film disk at many angles including non-Brewster""s angles of an absorbing layer of the thin film.
2. Description of Background Art
Coated thin film disks are used in a variety of industries. One example is the computer hard disk industry. A computer hard disk (magnetic storage device) is a non-volatile memory device that can store large amounts of data. One problem that the manufacturers of hard disks experience is how to maximize the operating life of a hard disk. When a hard disk fails the data stored therein may be difficult, expensive, or impossible to retrieve.
A schematic of a thin film disk used in magnetic storage devices is shown in FIG. 1. It includes a magnetic thin film (layer) 106 which is deposited upon a substrate 108 (typically a NiP plated Alxe2x80x94Mg alloy or glass). The magnetic thin film 106 can be protected by a thin film of carbon 104 (carbon layer), for example, whose thickness is typically 50 to 200 Angstroms (xc3x85) The carbon layer 104 is typically coated with a thin layer (10 to 30 Angstroms) of a fluorocarbon lubricant 102 (lubricant layer). The lubricant layer 102 serves to increase the durability of the underlying carbon layer 104 particularly when the magnetic read/write head contacts the disk, for example when the disk drive is turned off, as described below. During the development and testing of thin film disks it is necessary to subject thin film magnetic disks to numerous starts and stops of the read/write head. The start/stops cause the read/write head to contact the thin film disk 100 in a dedicated region of the thin film disk 100 known as the start/stop zone. The action of stopping and starting the thin film head on the start/stop zone can cause depletion and/or degradation of the fluorocarbon lubricant layer 102, wear of the carbon layer 104 and changes in the surface roughness. A conventional technique for measuring thin film characteristics are discussed in U.S. Pat. No. 4,873,430 which is incorporated by reference herein in its entirety. This patent describes a technique that uses a P polarized collimated (unfocussed) laser propagating at the Brewster""s angle of the film to measure film thickness and surface roughness.
U.S. Pat. No. 5,726,455 describes an optical system for measuring only the specular component of light reflected from a thin film magnetic disk. The patent purports that the system is able to measure lubricant coating thickness and coating wear. This system uses a temperature stabilized (Peltier effect cooled) light source and an integrating sphere detector which is remotely located from the disk. The angle of incidence is between the Brewster""s angle of the lubricant and that of the adjacent layer. This invention makes no provision for the measurement of the scattered light nor does it measure surface roughness.
Other techniques for measuring surface roughness are discussed in U.S. Pat. Nos. 5,608,527, 5,196,906, 5,313,542, 4,668,860, 5,406,082 and in the book xe2x80x9cOptical Scattering-Measurement and Analysisxe2x80x9d second edition by John C. Stover, SPIE Press, Bellingham, Wash., 1995 on page 169 through 170, which are all incorporated by reference herein in their entirety. These references relate to obtaining the surface roughness and do not address identifying lubricant thickness and degradation or thin film thickness or wear.
Specifically, U.S. Pat. No. 5,608,127 describes a technique for measuring the specular and scattered light in one scattering plane by using a multi-segmented array. The specular and scattered lights are used to obtain the surface roughness. U.S. Pat. No. 5,196,906 describes a modular scatterometer for determining surface roughness from an array of detectors positioned along a hemisphere. U.S. Pat. No. 5,313,542 describes a scatterometer which uses depolarized light from a laser diode and fiber optic bundles to collect partial or full hemispherically scattered light. U.S. Pat. No. 4,668,860 describes a scatterometer for evaluating the surface quality of an optical element which has both bulk and surface scatter. This patent describes a technique that purports to separate surface and bulk scatter by using the polarization characteristics of the light. U.S. Pat. No. 5,406,082 describes a surface inspection and characterization system that uses a broadband infrared light source which is directed onto the surface to be inspected. The reflected light is separated into several wavelengths and these signals are used to characterize the surface by such properties as absorbency.
A technique for combining the measurement of thin film thickness and surface roughness is described in a brochure by AHEAD Optoelectronics, Inc., Taipei, Taiwan, R.O.C, which is incorporated by reference herein in its entirety. This describes an instrument called an Integrating Sphere Ellipsometry Analyzer. This instrument is a combined ellipsometer and integrating sphere analyzer. This brochure teaches a measurement technique that uses an ellipsometric technique for the ex situ measurement of absolute film thickness and indices of refraction. This technique also uses an integrating sphere to measure surface microroughness at a variable angle. The system as described is designed for ex situ measurement of film thickness and surface microroughness, it is not capable of measuring in situ wear, lubricant and surface roughness.
A technique for measuring thin film properties at Brewster""s angle is described in S. Meeks et. al., Optical Surface Analysis of the Head-Disk-Interface of Thin Film Disks, ASME Transactions on Tribology, Vol. 117, pp. 112-118, (January 1995), which is incorporated by reference herein in its entirety.
None of these references teach a single system and method for performing all of these measurements in situ. In addition, references Meeks et. al. and Juliana et al. teach that the measurement should occur at substantially Brewster""s angle of the carbon 104. U.S. Pat. No. 5,726,455 teaches that the measurement should occur between Brewster""s angle of the lubricant and that of the adjacent layer. A stated benefit of using this angle is that the light signal will not reflect off of the carbon 104 and instead will pass directly through the carbon 104 and reflect off of the magnetic layer 106.
What is needed is a system and method for examining thin film disks that: (1) measures the amount of lubricant thickness and thickness change; (2) measures the extent of lubricant degradation; (3) measures the wear and thickness of the carbon layer; (4) measures the absolute surface roughness and changes in the surface roughness; (5) performs magnetic imaging; (6) performs optical profiling; and (7) enables these measurements to be (a) performed simultaneously, (b) performed at an angle of incidence that is substantially different from Brewster""s angle, and (c) performed in situ or ex situ.
The invention is a system and method for measuring thin film disk properties using an optical system that transmits electromagnetic radiation toward the thin film disk at an angle of incidence that need not be substantially Brewster""s angle. The present invention measures lubricant thickness and degradation, carbon wear and thickness, and surface roughness of thin film magnetic disks at angles that are not substantially Brewster""s angle of the thin film protective overcoat (carbon). A focused optical light whose polarization can be switched between P or S polarization is incident at an angle to the surface of the thin film magnetic disk. This allows the easy measurement of the change in lubricant thickness due to the interaction of the thin film head, the absolute lubricant thickness and degradation of the lubricant. It also allows the measurement of changes in carbon thickness and the absolute carbon thickness. The surface roughness can also be measured.